Science X

Syllabus

Carbon and its compounds, Periodic Classification of Elements, How do Organisms Reproduce, Heredity & Evolution, Electricity, Magnetic Effect of current, Our Environment, Management of Natural Resources.

4-Carbon & Its Compounds

1. What would be the electron dot structure of carbon dioxide which has the formula CO2 ?

2. What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint – The eight atoms of sulphur are joined together in the form of a ring.)

1. How many structural isomers can you draw for pentane?

2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

3. What will be the formula and electron dot structure of cyclopentane?

4. Draw the structures for the following compounds.

(i) Ethanoic acid

(ii) Bromopentane*

(iii) Butanone

(iv) Hexanal.

Are structural isomers possible for bromopentane?

5. How would you name the following compounds?

(i) CH3—CH2—Br

(ii) H-CH=O

(iii) CH3CH2CH2CH2C=CH

1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

1. How would you distinguish experimentally between an alcohol and a carboxylic acid?

2. What are oxidising agents?

1. Would you be able to check if water is hard by using a detergent?

2. People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

1. Ethane, with the molecular formula C2H6 has

(a) 6 covalent bonds.

(b) 7 covalent bonds.

(d) 9 covalent bonds.

2. Butanone is a four-carbon compound with the functional group

(a) carboxylic acid.

(b) aldehyde.

(d) alcohol.

3. While cooking, if the bottom of the vessel is getting blackened on the outside, it means that

(a) the food is not cooked completely.

(b) the fuel is not burning completely.

(d) the fuel is burning completely

4. Explain the nature of the covalent bond using the bond formation in CH3Cl.

5. Draw the electron dot structures for (a) ethanoic acid. (b) H2S. (c) propanone. (d) F2 .

6. What is an homologous series? Explain with an example.

7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?

9. Why are carbon and its compounds used as fuels for most applications?

10. Explain the formation of scum when hard water is treated with soap.

11. What change will you observe if you test soap with litmus paper (red and blue)?

12. What is hydrogenation? What is its industrial application?

13. Which of the following hydrocarbons undergo addition reactions:

C2H6, C3H8, C3H6, C2H2 and CH4.

14. Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.

15. Explain the mechanism of the cleaning action of soaps.

5-Periodic Classification of Elements

1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

2. What were the limitations of Döbereiner’s classification?

3. What were the limitations of Newlands’ Law of Octaves?

1. Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements:

K, C, AI, Si, Ba.

2. Besides gallium, which other elements have since been discovered that were left by Mendeléev in his Periodic Table? (any two)

3. What were the criteria used by Mendeléev in creating his Periodic Table?

4. Why do you think the noble gases are placed in a separate group?

1. How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?

2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

3. Name (a) three elements that have a single electron in their outermost shells.

(b) two elements that have two electrons in their outermost shells.

4. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?

(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?

5. In the Modern Periodic Table, which are the metals among the first ten elements?

6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic? Ga Ge As Se Be

1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table.

(a) The elements become less metallic in nature.

(b) The number of valence electrons increases.

(d) The oxides become more acidic.

2. Element X forms a chloride with the formula XCl2 , which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as (a) Na (b) Mg (c) AI (d) Si

3. Which element has (a) two shells, both of which are completely filled with electrons? (b) the electronic configuration 2, 8, 2? (c) a total of three shells, with four electrons in its valence shell? (d) a total of two shells, with three electrons in its valence shell? (e) twice as many electrons in its second shell as in its first shell?

4. (a) What property do all elements in the same column of the Periodic Table as boron have in common? (b) What property do all elements in the same column of the Periodic Table as fluorine have in common?

5. An atom has electronic configuration 2, 8, 7.

(a) What is the atomic number of this element?

(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.) N(7) F(9) P(15) Ar(18)

6. The position of three elements A, B and C in the Periodic Table are shown below –

Group 16 .....Group 17

.....................................

..............................A.....

.....................................

....B........................C..

(a) State whether A is a metal or non-metal.

(b) State whether C is more reactive or less reactive than A.

(d) Which type of ion, cation or anion, will be formed by element A?

7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table? 9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium? 10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.

8 - How do Organisms Reproduce?

Q1. What is the importance of DNA copying in reproduction?

Ans. The copying of DNA during reproduction is important because:

The replication of DNA ensures that each daughter cell formed at the end of cell division, receives equal amount of DNA.

If DNA won't be copied then the daughter cells won't receive all necessary genes.

There may be absence of important proteins which are encoded by DNA, if DNA won't be replicated. Copying of DNA may result in recombination which will lead to evolution. Transfer of genetic material from one generation to other makes the organisms to have similar functions. Stability of population is maintained.

Q2. Why is variation beneficial to the species but not necessarily for the individual?

Ans. Species occupy well-defined spaces in the ecosystem, called niches, by increasing their population through reproduction. These niches in the ecosystem may change upon time, even drastically, which are not in control of these organisms occupying the space.

The changes, such as an increase in temperature, may cause a threat to the survival of the species in that niche. If the species have variations within the population, some of the variants might be able to cope with the change, thus having a survival advantage.

Thus, these variants will survive, leaving behind the individuals in the population with no desirable variation. Thus, these variants help in the survival of species. But, not all variations are beneficial for individual organisms.

Q1. How does binary fission differ from multiple fission?

Ans. In binary fission, the parent cell divides itself into two equal and identical daughter cells.

In multiple fission, a single parent cell is divided into many daughter cells. It is the most common form of reproduction in protists and in some parasitic species.

Q2. How will an organism be benefited if it reproduces through spores?

Ans. Reproduction through spores gives several advantages to an organism. Some of them are as follows:

- Spores can remain dormant till favourable conditions become available.

- Spores help an organism to tide over the bad phase.

- Spores can be spread through water, air or animals and thus is good for the spread of an organism to more places.

Q3. Can you think of reasons why more complex organisms cannot give rise to new individuals through regeneration?

Ans. Simple organisms are capable of producing new organisms through the process of regeneration. This is possible in simple organisms because the entire body is made up of similar kind of cells in which any part of their body can be formed by growth and development.

In case of the complex organisms have organ system level organization. all the organ systems of their body work together as an interconnected unit.

The tissues in complex organisms cannot regenerate a new individual as they are highly differentiated to perform specialized functions. For example, human skin cannot regenerate into a new individual as it is a highly differentiated tissue performing a designed function.

Q4. Why is vegetative propagation practised for growing some types of plants?

Ans. Vegetative propagation is practiced because:

1. It is more rapid and easier method.

2. It can help plant to grow in regions where germination failed.

3. It is useful for propagating those plants that do not produce viable seeds or produce less number of seeds.

4.. it is quick method of producing several plants in short period of time..

5. Example- Tuber of potato, the rhizome of ginger.

Q5. Why is DNA copying an essential part of the process of reproduction?

Ans. The passing on of the traits from the parents to the offspring is called heredity.

DNA replication is a process by which the cells of an organism make additional copies of DNA.

These additional copies of DNA can be transferred to the offspring which carries the genes of characters of both the parents (father and mother).

Thus, DNA copying is an essential part of reproduction.

Q1. How is the process of pollination different from fertilisation?

Ans. The process of pollination is simply the transfer of pollen from one flower to other onto the stigma. The process of fertilization is what happens after the pollen is transferred into the stigma, that is, when the male and female gametes from the zygote.

Pollination takes place only in flowering plants whereas, fertilization happens in almost all animals and plants.

Q2. What is the role of the seminal vesicles and the prostate gland?

Ans. The function of prostate gland is to secrete a slightly alkaline fluid, milky or white in appearance, which protects the sperm from the acidic environment in female vaginal tract. Also, provide nourishment and energy to the sperm.

Seminal vesicles secrete a significant proportion of the fluid that ultimately becomes the semen when mixed with sperm.

Q3. What are the changes seen in girls at the time of puberty?

Ans. At the time of puberty secondary sexual characters, develop and sexual maturity takes place in both girls and boys. The changes that seen in girls at the time of puberty are as follows:

i. Voice becomes thin

II. Menstruation begins

iii. Breast enlarge

iv. Hips become rounded

v. Hair grows under the armpit and in the pubic area

Q4. How does the embryo get nourishment inside the mother’s body?

Ans. The embryo gets nourishment inside the mother body through a special tissue called placenta. The embryo grows inside the mother's womb and gets nourishment from mother's blood through the tissue called placenta. The placenta is a temporary organ that develops in the uterus during pregnancy. It attaches the fetus to the uterine wall. It also provides nutrients to the fetus and also allows the fetus to transfer waste product's to the mother's blood.

Q5. If a woman is using a copper -T, will it help in protecting her from sexually transmitted diseases?

1. Asexual reproduction takes place through budding in (a) amoeba. (b) yeast. (c) plasmodium. (d) leishmania.

2. Which of the following is not a part of the female reproductive system in human beings? (a) Ovary (b) Uterus (c) Vas deferens (d) Fallopian tube

3. The anther contains (a) sepals. (b) ovules. (c) pistil. (d) pollen grains.

4. What are the advantages of sexual reproduction over asexual reproduction?

5. What are the functions performed by the testis in human beings?

6. Why does menstruation occur? 7. Draw a labelled diagram of the longitudinal section of a flower.

8. What are the different methods of contraception?

9. How are the modes for reproduction different in unicellular and multicellular organisms?

10. How does reproduction help in providing stability to populations of species?

11. What could be the reasons for adopting contraceptive methods?

9 - Heredity & Evolution

1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?

2. How does the creation of variations in a species promote survival?

1. How do Mendel’s experiments show that traits may be dominant or recessive?

2. How do Mendel’s experiments show that traits are inherited independently?

3. A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?

4. How is the sex of the child determined in human beings?

1. What are the different ways in which individuals with a particular trait may increase in a population?

2. Why are traits acquired during the life-time of an individual not inherited?

3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?

1. What factors could lead to the rise of a new species?

2. Will geographical isolation be a major factor in the speciation of a self pollinating plant species? Why or why not?

3. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?

1. Give an example of characteristics being used to determine how close two species are in evolutionary terms.

2. Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?

3. What are fossils? What do they tell us about the process of evolution?

1. Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species?

2. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?

1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as (a) TTWW (b) TTww (c) TtWW (d) TtWw

2. An example of homologous organs is (a) our arm and a dog’s fore-leg. (b) our teeth and an elephant’s tusks. (c) potato and runners of grass. (d) all of the above.

3. In evolutionary terms, we have more in common with (a) a Chinese school-boy. (b) a chimpanzee. (c) a spider. (d) a bacterium.

4. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?

5. How are the areas of study – evolution and classification – interlinked?

6. Explain the terms analogous and homologous organs with examples.

7. Outline a project which aims to find the dominant coat colour in dogs.

8. Explain the importance of fossils in deciding evolutionary relationships.

9. What evidence do we have for the origin of life from inanimate matter?

10. Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How does this affect the evolution of those organisms that reproduce sexually?

11. How is the equal genetic contribution of male and female parents ensured in the progeny?

12. Only variations that confer an advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?

12-Electricity

Example 12.1 A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes. Find the amount of electric charge that flows through the circuit.

Solution We are given, I = 0.5 A; t = 10 min = 600 s. From Eq. (12.1), we have Q = It = 0.5 A × 600 s = 300 C

1. What does an electric circuit mean?

2. Define the unit of current.

3. Calculate the number of electrons constituting one coulomb of charge.

Example 12.2 How much work is done in moving a charge of 2 C across two points having a potential difference 12 V?

Solution The amount of charge Q, that flows between two points at potential difference V (= 12 V) is 2 C. Thus, the amount of work W, done in moving the charge [from Eq. (12.2)] is W = VQ = 12 V × 2 C = 24 J.

1. Name a device that helps to maintain a potential difference across a conductor.

2. What is meant by saying that the potential difference between two points is 1 V?

3. How much energy is given to each coulomb of charge passing through a 6 V battery?

Example 12.3 (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω? (b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?

Solution (a) We are given V = 220 V; R = 1200 Ω. From Eq. (12.6), we have the current I = 220 V/1200 Ω = 0.18 A.

(b) We are given, V = 220 V, R = 100 Ω. From Eq. (12.6), we have the current I = 220 V/100 Ω = 2.2 A. Note the difference of current drawn by an electric bulb and electric heater from the same 220 V source!

Example 12.4 The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the potential difference is increased to 120 V?

Solution We are given, potential difference V = 60 V, current I = 4 A. According to Ohm’s law, 60 V = = 15 4 A V R I = Ω . When the potential difference is increased to 120 V the current is given by current = 120 V = = 8 A 15 V R Ω . The current through the heater becomes 8 A.

Example 12.5 Resistance of a metal wire of length 1 m is 26 Ω at 20°C. If the diameter of the wire is 0.3 mm, what will be the resistivity of the metal at that temperature? Using Table 12.2, predict the material of the wire.

Solution We are given the resistance R of the wire = 26 Ω, the diameter d = 0.3 mm = 3 × 10-4 m, and the length l of the wire = 1 m.

Therefore, from Eq. (12.10), the resistivity of the given metallic wire is ρ = (RA/l) = (Rπd 2/4l ) Substitution of values in this gives ρ = 1.84 × 10–6 Ω m

The resistivity of the metal at 20°C is 1.84 × 10–6 Ω m. From Table 12.2, we see that this is the resistivity of manganese.

Example 12.6 A wire of given material having length l and area of cross-section A has a resistance of 4 Ω. What would be the resistance of another wire of the same material having length l/2 and area of cross-section 2A?

Solution For first wire R1 = ρ l A = 4Ω Now for second wire R2 ρ /2 = 2 l A = ρ 1 4 l A R2 1 = 4 R1 R2 = 1Ω The resistance of the new wire is 1Ω.

1. On what factors does the resistance of a conductor depend?

2. Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?

3. Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?

4. Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?

5. Use the data in Table 12.2 to answer the following –

(a) Which among iron and mercury is a better conductor?

(b) Which material is the best conductor?

Example 12.7 An electric lamp, whose resistance is 20 Ω, and a conductor of 4 Ω resistance are connected to a 6 V battery (Fig. 12.9). Calculate (a) the total resistance of the circuit, (b) the current through the circuit, and (c) the potential difference across the electric lamp and conductor.

Solution The resistance of electric lamp, R1 = 20 Ω, The resistance of the conductor connected in series, R2 = 4 Ω. Then the total resistance in the circuit R = R1 + R2 Rs = 20 Ω + 4 Ω = 24 Ω. The total potential difference across the two terminals of the battery V = 6 V. Now by Ohm’s law, the current through the circuit is given by I = V/Rs = 6 V/24 Ω = 0.25 A.

1. Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor, and a plug key, all connected in series.

2. Redraw the circuit of Question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure the potential difference across the 12 Ω resistor. What would be the readings in the ammeter and the voltmeter?

Example 12.8 In the circuit diagram given in Fig. 12.10, suppose the resistors R1 , R2 and R3 have the values 5 Ω, 10 Ω, 30 Ω, respectively, which have been connected to a battery of 12 V. Calculate (a) the current through each resistor, (b) the total current in the circuit, and (c) the total circuit resistance.

Solution R1 = 5 Ω, R2 = 10 Ω, and R3 = 30 Ω. Potential difference across the battery, V = 12 V. This is also the potential difference across each of the individual resistor; therefore, to calculate the current in the resistors, we use Ohm’s law. The current I 1 , through R1 = V/ R1 I 1 = 12 V/5 Ω = 2.4 A.

Example 12.9 If in Fig. 12.12, R1 = 10 Ω, R2 = 40 Ω, R3 = 30 Ω, R4 = 20 Ω, R5 = 60 Ω, and a 12 V battery is connected to the arrangement. Calculate (a) the total resistance in the circuit, and (b) the total current flowing in the circuit.

Solution Suppose we replace the parallel resistors R1 and R2 by an equivalent resistor of resistance, R′. Similarly we replace the parallel resistors R3 , R4 and R5 by an equivalent single resistor of resistance R″. Then using Eq. (12.18), we have 1/ R′ = 1/10 + 1/40 = 5/40; that is R′ = 8 Ω.

Similarly, 1/ R″ = 1/30 + 1/20 + 1/60 = 6/60; that is, R″ = 10 Ω. Thus, the total resistance, R = R′ + R″ = 18 Ω. To calculate the current, we use Ohm’s law, and get I = V/R = 12 V/18 Ω = 0.67 A.

1. Judge the equivalent resistance when the following are connected in parallel – (a) 1 Ω and 106 Ω, (b) 1 Ω and 103 Ω, and 106 Ω.

2. An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter of resistance 500 Ω are connected in parallel to a 220 V source. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?

3. What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?

4. How can three resistors of resistances 2 Ω, 3 Ω, and 6 Ω be connected to give a total resistance of (a) 4 Ω, (b) 1 Ω?

5. What is (a) the highest, (b) the lowest total resistance that can be secured by combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?

Example 12.10 An electric iron consumes energy at a rate of 840 W when heating is at the maximum rate and 360 W when the heating is at the minimum. The voltage is 220 V. What are the current and the resistance in each case?

Solution From Eq. (12.19), we know that the power input is P = V I Thus the current I = P/V (a) When heating is at the maximum rate, I = 840 W/220 V = 3.82 A; and the resistance of the electric iron is R = V/I = 220 V/3.82 A = 57.60 Ω. (b) When heating is at the minimum rate, I = 360 W/220 V = 1.64 A; and the resistance of the electric iron is R = V/I = 220 V/1.64 A = 134.15 Ω.

Example 12.11 100 J of heat is produced each second in a 4 Ω resistance. Find the potential difference across the resistor.

Solution H = 100 J, R = 4 Ω, t = 1 s, V = ? From Eq. (12.21) we have the current through the resistor as I = √(H/Rt) = √[100 J/(4 Ω × 1 s)] = 5 A Thus the potential difference across the resistor, V [from Eq. (12.5)] is V = IR = 5 A × 4 Ω = 20 V. Figure 12.13 A steady current in a purely resistive electric circuit 2.

1. Why does the cord of an electric heater not glow while the heating element does?

2. Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V.

3. An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.

1 kW h = 1000 watt × 3600 second = 3.6 × 106 watt second = 3.6 × 106 joule (J)

Example 12.12 An electric bulb is connected to a 220 V generator. The current is 0.50 A. What is the power of the bulb?

Solution P = VI = 220 V × 0.50 A = 110 J/s = 110 W. Example 12.13 An electric refrigerator rated 400 W operates 8 hour/day. What is the cost of the energy to operate it for 30 days at Rs 3.00 per kW h?

Solution The total energy consumed by the refrigerator in 30 days would be 400 W × 8.0 hour/day × 30 days = 96000 W h = 96 kW h

Thus the cost of energy to operate the refrigerator for 30 days is 96 kW h × Rs 3.00 per kW h = Rs 288.00

1. What determines the rate at which energy is delivered by a current?

2. An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.

1. A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R′, then the ratio R/R′ is – (a) 1/25 (b) 1/5 (c) 5 (d) 25

2. Which of the following terms does not represent electrical power in a circuit? (a) I 2R (b) IR2 (c) VI (d) V 2/R

3. An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be – (a) 100 W (b) 75 W (c) 50 W (d) 25 W

4. Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be – (a) 1:2 (b) 2:1 (c) 1:4 (d) 4:1

5. How is a voltmeter connected in the circuit to measure the potential difference between two points?

6. A copper wire has diameter 0.5 mm and resistivity of 1.6 × 10–8 Ω m. What will be the length of this wire to make its resistance 10 Ω? How much does the resistance change if the diameter is doubled?

7. The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below – I (amperes) 0.5 1.0 2.0 3.0 4.0 V (volts) 1.6 3.4 6.7 10.2 13.2 Plot a graph between V and I and calculate the resistance of that resistor.

8. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.

9. A battery of 9 V is connected in series with resistors of 0.2 Ω, 0.3 Ω, 0.4 Ω , 0.5 Ω and 12 Ω, respectively. How much current would flow through the

12 Ω resistor? 10. How many 176 Ω resistors (in parallel) are required to carry 5 A on a 220 V line?

11. Show how you would connect three resistors, each of resistance 6 Ω, so that the combination has a resistance of (i) 9 Ω, (ii) 4 Ω.

12. Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A?

13. A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24 Ω resistance, which may be used separately, in series, or in parallel. What are the currents in the three cases?

14. Compare the power used in the 2 Ω resistor in each of the following circuits: (i) a 6 V battery in series with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors.

13 - Magnetic Effects of Electric Current

1. Why does a compass needle get deflected when brought near a bar magnet?

Example 13.1 A current through a horizontal power line flows in east to west direction. What is the direction of magnetic field at a point directly below it and at a point directly above it?

Solution The current is in the east-west direction. Applying the right-hand thumb rule, we get that the magnetic field (at any point below or above the wire) turns clockwise in a plane perpendicular to the wire, when viewed from the east end, and anti-clockwise, when viewed from the west end.

1. Draw magnetic field lines around a bar magnet.

2. List the properties of magnetic field lines.

3. Why don’t two magnetic field lines intersect each other?

1. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

2. The magnetic field in a given region is uniform. Draw a diagram to represent it.

3. Choose the correct option. The magnetic field inside a long straight solenoid-carrying current (a) is zero. (b) decreases as we move towards its end. (c) increases as we move towards its end. (d) is the same at all points.

Example 13.2 An electron enters a magnetic field at right angles to it, as shown in Fig. 13.14. The direction of force acting on the electron will be (a) to the right. (b) to the left. (c) out of the page. (d) into the page.

Solution Answer is option (d). The direction of force is perpendicular to the direction of magnetic field and current as given by Fleming’s left hand rule. Recall that the direction of current is taken opposite to the direction of motion of electrons. The force is therefore directed into the page.

1. Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.) (a) mass (b) speed (c) velocity (d) momentum

2. In Activity 13.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?

3. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is (a) towards south (b) towards east (c) downward (d) upward

1. State Fleming’s left-hand rule.

2. What is the principle of an electric motor?

3. What is the role of the split ring in an electric motor?

1. Explain different ways to induce current in a coil.

1. State the principle of an electric generator.

2. Name some sources of direct current.

3. Which sources produce alternating current?

4. Choose the correct option. A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each

(a) two revolutions

(b) one revolution

(d) one-fourth revolution

1. Name two safety measures commonly used in electric circuits and appliances.

2. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.

3. What precaution should be taken to avoid the overloading of domestic electric circuits?

1. Which of the following correctly describes the magnetic field near a long straight wire? (a) The field consists of straight lines perpendicular to the wire. (b) The field consists of straight lines parallel to the wire. (c) The field consists of radial lines originating from the wire. (d) The field consists of concentric circles centred on the wire.

2. The phenomenon of electromagnetic induction is (a) the process of charging a body. (b) the process of generating magnetic field due to a current passing through a coil. (c) producing induced current in a coil due to relative motion between a magnet and the coil. (d) the process of rotating a coil of an electric motor. 3. The device used for producing electric current is called a (a) generator. (b) galvanometer. (c) ammeter. (d) motor.

4. The essential difference between an AC generator and a DC generator is that (a) AC generator has an electromagnet while a DC generator has permanent magnet. (b) DC generator will generate a higher voltage. (c) AC generator will generate a higher voltage. (d) AC generator has slip rings while the DC generator has a commutator.

5. At the time of short circuit, the current in the circuit (a) reduces substantially. (b) does not change. (c) increases heavily. (d) vary continuously.

6. State whether the following statements are true or false. (a) An electric motor converts mechanical energy into electrical energy. (b) An electric generator works on the principle of electromagnetic induction. (c) The field at the centre of a long circular coil carrying current will be parallel straight lines. (d) A wire with a green insulation is usually the live wire of an electric supply.

7. List two methods of producing magnetic fields.

8. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.

9. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?

10. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?

12. Name some devices in which electric motors are used.

13. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?

14. Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

15. State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.

16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

17. When does an electric short circuit occur? 18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?

15 - Our Environment

1. What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

2. What is the role of decomposers in the ecosystem?

1. Why are some substances biodegradable and some non-biodegradable?

2. Give any two ways in which biodegradable substances would affect the environment.

3. Give any two ways in which non-biodegradable substances would affect the environment.

1. What is ozone and how does it affect any ecosystem?

2. How can you help in reducing the problem of waste disposal? Give any two methods.

1. Which of the following groups contain only biodegradable items? (a) Grass, flowers and leather (b) Grass, wood and plastic (c) Fruit-peels, cake and lime-juice (d) Cake, wood and grass

2. Which of the following constitute a food-chain? (a) Grass, wheat and mango (b) Grass, goat and human (c) Goat, cow and elephant (d) Grass, fish and goat

3. Which of the following are environment-friendly practices? (a) Carrying cloth-bags to put purchases in while shopping (b) Switching off unnecessary lights and fans (c) Walking to school instead of getting your mother to drop you on her scooter (d) All of the above

4. What will happen if we kill all the organisms in one trophic level?

5. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?

6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

7. What are the problems caused by the non-biodegradable wastes that we generate?

8. If all the waste we generate is biodegradable, will this have no impact on the environment?

9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

16 - Sustainable Management of Natural Resources

1. What changes can you make in your habits to become more environmentfriendly?

Ans. ∙Using the 3R's philosophy (Reduce, Reuse, and Recycle).

∙Turn off lights and fans that aren't in use.

∙Using unleaded petrol in automobiles.

∙Taking public transportation, such as buses or trains

∙Forests must be preserved in order for animals to survive.

∙Reduce the amount of burning plastic.

2. What would be the advantages of exploiting resources with short-term aims?

Ans. If resources are exploited with short-term aims it will be very beneficial for the present generation as it will help in the growth of the economy at a faster rate.

3. How would these advantages differ from the advantages of using a longterm perspective in managing our resources?

Ans. A long term perspective in managing our resources would always be better than a short term perspective. A long term perspective helps us in protecting our environment and also in conserving our natural resources. This is an integral part of sustainable development. This will help us in ensuring that the future generations also get to enjoy the bounty of nature.

4. Why do you think that there should be equitable distribution of resources? What forces would be working against an equitable distribution of our resources?

Ans. Natural resources of the earth must be distributed among the people equally so that each and everyone gets benefits of the resources. Some forces work against the humanity like greed, corruption and the lobby of the rich and powerful hinder equitable distribution of resources.

1. Why should we conserve forests and wildlife?

Ans. → Forests provide us oxygen and responsible for rainfall.

→ Forests prevent soil erosion.

→ Plants are dependent on animals and birds for their pollination and seed dispersal and maintain energy flow in an ecosystem.

→ Forests provide us various medicines.

→ Various forest products are used as raw materials in industries.

2. Suggest some approaches towards the conservation of forests.

Ans. Some approaches towards the conservation of forests are:

→ Conversion of forests into national parks, sanctuaries

→ Coordinating efforts with local people in conserving the forest.

→ Preventing the destruction of forests for making roads, dams, hotels etc.

→ Increasing plantation.

1. Find out about the traditional systems of water harvesting/ management in your region.

Ans.

2. Compare the above system with the probable systems in hilly/ mountainous areas or plains or plateau regions.

3. Find out the source of water in your region/locality. Is water from this source available to all people living in that area?

1. What changes would you suggest in your home in order to be environment-friendly?

2. Can you suggest some changes in your school which would make it environmentfriendly?

3. We saw in this chapter that there are four main stakeholders when it comes to forests and wildlife. Which among these should have the authority to decide the management of forest produce? Why do you think so?

4. How can you as an individual contribute or make a difference to the management of (a) forests and wildlife, (b) water resources and (c) coal and petroleum?

5. What can you as an individual do to reduce your consumption of the various natural resources?

6. List five things you have done over the last one week to — (a) conserve our natural resources. (b) increase the pressure on our natural resources.

7. On the basis of the issues raised in this chapter, what changes would you incorporate in your lifestyle in a move towards a sustainable use of our resources?